Sputtering is a method of depositing thin metal and insulating films onto semiconductor wafers. Sputtering is a term used to describe a physical mechanism in which atoms are dislodged from the surface of a material by collision with high energy particles. It can be compared to throwing steel balls at a concrete wall. Upon impact, the ball tears away fragments of the concrete, resulting in fragments which retain the chemical and physical properties of the concrete. If the process is continued, surfaces in the vicinity of the impact are covered with a layer of concrete dust. In sputtering, the "steel balls" are ionized atoms, typically argon, and the "wall" is a plate of material to be sputtered, commonly referred to as the "target".
Sputtering takes place in an evacuated chamber. Ionized argon atoms are introduced into the chamber which contains the wafers and a target of the film material to be sputtered. In diode sputtering, the target is maintained at a negative potential relative to the positively charged argon atom. This causes the positive ion to be accelerated towards the negatively charged target. The argon atoms slam into the target, thus tearing off some of the target material. Since the chamber is maintained at a vacuum, the liberated material settles on everything in the chamber, including wafers positioned therein. Two kinds of diode sputtering methods include direct current and radio frequency sputtering. Other sputtering methods include the triode and magnetron methods. Regardless of the method, ionized atoms are caused to accelerate toward the target, thus shearing material therefrom at the atomic level and depositing very thin films on semiconductor wafers and everything else in the sputtering chamber.
One drawback of sputtering is the difficulty in achieving conformal coverage deep within high aspect ratio steps or contacts on semiconductor wafers. The problem is described with reference to FIG. 1. Such illustrates a semiconductor wafer fragment 10 comprised of a bulk substrate material 12 having a deep contact opening 14 formed therein. In sputtering, the trajectory of the atomic material sputtered from the target depends in significant part upon the incident angle of the bombarding atom. Accordingly, atoms directed towards the wafer surface come from varying angles, only a comparatively small portion of which arrive at the wafer surface in a substantially perpendicular angle. The downwardly directed arrows shown in FIG. 1 are representative of the varying angles with which the depositing atoms strike the wafer. Such provides an undesired drawback of not conformally coating deep within contacts, as the quantity of perpendicularly angled atoms is small in comparison to the other angled atoms. This results in what is commonly known as a bread-loafing, non-conformal deposit of a metal layer 16, as shown.
To overcome such drawback, a device known as a collimator is used, such as is diagrammatically represented in FIG. 2 with reference numeral 18. Collimator 18 typically comprises a disk-shaped object having a plurality of round holes or openings 20 provided therethrough. A collimator functions effectively as a filter, essentially allowing only the perpendicularly angled atoms to pass therethrough and coat wafer 12. This results in a more conformal deposition within deep contacts than is possible when a collimator is not used. Layer 17 depicts such a deposited layer.
Collimators, like everything else within a sputtering chamber, of course also get coated with the sputtering material. Such results in the effect illustrated by FIG. 3. Collimator 18 gets coated with the sputtered deposited material 16 producing balled-up gatherings 16a of material 16. This at some point effectively diminishes the diametric size of openings 20 to where the deposition rate on a wafer 12 becomes unsatisfactory. Existing techniques involving collimators require them to be removed and cleaned such that sputtering can continue in a desired manner and rate.
Unfortunately, it is a tedious and costly job of cleaning collimators. The sputtering chamber must be effectively shut down and vacuum broken such that the collimators can be removed for cleaning. Presently, the service lifetime of a collimator is typically one-third that of the typical replacement life for a target in sputter deposition methods in semiconductor wafer processing.
It would be desirable to provide methods or techniques for increasing collimator lifetime to a point where such at least equals that of the target lifetime, enabling coincident servicing of the target and collimator.